| DE3307796A | ||||
| DE3423285A | ||||
| FR2484990A | ||||
| NL8602150A | ||||
| 4372856 | Process and system for anaerobic treatment of waste |
The invention relates to the removal of hydrogen sulphide (H2S) from biogas.
Hydrogen sulphide is a disturbing but hardly avoidable component of biogas. A process frequently used for removing hydrogen sulphide from biogas is scrubbing the gas with an aqueous liquid having an increased pH. This increased pH can be adjusted by the addition of caustic soda or other agents. Such processes are known for example from European Patent Applications 229,587 and 331,806.
The efficiences that can be achieved with such processes vary from 50 to 99.9%, depending on the amount of hydroxide added and the capacity of the apparatus.
A drawback of scrubbers of the type is this high consumption of chemicals, resulting in high operational costs.
According to the present invention, the natural alkalinity generated during the aerobic biological purification of waste water is used instead of added alkaline chemicals (for example caustic soda).
Alkalinity is to be understood here as the total of negative ions and neutral particles that can dissociate H2S.
This natural alkalinity can arise for example in the following two manners:
This causes the pH to increase.
This results in a marked pH increase.
By stripping carbon dioxide during the aerobic post-treatment as indicated at (a), the pH value increases further.
The aerobically treated waste water having alkalinity obtained in a natural way is contacted with the biogas containing H2S. The waste water also contains biomass. The H2S will be absorbed from the biogas into the aqueous phase.
The efficiencies that may be obtained in this way vary from 50 to 95%, depending on the water flow/gas flow ratio and the volume of the apparatus. For a H2S removal rate of 50%, a water flow/gas flow ratio of 0.1 is generally sufficient. For higher efficiencies, a ratio of at least 0.2 and in particular 0.5 or higher can be chosen, depending on the composition of the waste water and the biogas.
As a result of the charge neutrality, the amount of natural alkalinity in the form of OH- and HCO3- (in meq) generated during aerobic treatment is basically equal to the number of meq/l of cations (such as Na+, K+, Ca2+, Mg2+, etc.) present minus the number of free anions (such as Cl-, SO42-, etc.) present. Thus, a high salt concentration prior to the aerobic treatment can lead to an increased alkalinity after the aerobic treatment and CO2 stripping.
The great advantage of the present process is that no added chemicals are used, causing the operational costs to be low. A further advantage is that scrubbing liquid containing the H2S absorbed therein can be recycled to the aerobic treatment without difficulty and without further treatment. Another advantage is that the recycled washing liquid can serve to adjust the pH of the aerobic treatment, which may lead to further savings in chemicals.
The present process for removing H2S from biogas can be used not only at the site where the waste water is treated both anaerobically and aerobically, but also where a sludge fermentation is present in addition to an aerobic plant.
The biogas formed in the anaerobic phase usually contains, in addition to methane and other gases, 0.1 - 3% by volume of H2S and can be stripped of H2S according to the invention by scrubbing it with a water/biomass mixture. Reactions including the following occur then:
The water phase containing the H2S absorbed therein is recycled to the aerobic phase, where a biological oxidation takes place according to the following equation:
The aerobic effluent serving as scrubbing liquid can be taken from the aeration tank. The gas is scrubbed with a water/biomass mixture. This mixture containing absorbed H2S is recycled to the aeration tank. Figure 1 depicts the liquid flows according to this embodiment in diagram form. Herein (1) represents the anaerobic treatment, (2) is the aeration tank, (3) is the secondary settling and (4) is the biogas scrubber.
If clarified effluent would be used as a scrubbing liquid as shown in figure 2, wherein the reference numbers have the same meanings as in figure 1, a disadvantage would be that the secondary settling tank has a higher hydraulic load. The embodiment of figure 2 is not part of the invention.
An installation wherein the removal of H2S form biogas according to the process of the present invention can be carried out is depicted in figure 3. In addition to an inlet 11 and an outlet 12 for biogas and an inlet 13 and an outlet 14 for scrubbing liquid, this installation comprises a contact material 15 for improving the H2S transfer as well as a liquid collector 16. Inlet 13 and outlet 14 are connected with an aerobic reactor 2, which has a second inlet for waste water as shown in figure 1.
The process for removing H2S from biogas has been tested in a purification plant treating waste water of a brewery.
The biogas produced in the anaerobic reactor was scrubbed with a water/biomass mixture originating from the aerobic (activated sludge) treatment of the carousel type.
| Biogas data: | |
| - Flow | 150-225 m3/hr |
| - H2S concentration | 0.2-0.4% |
| - CO2 concentration | 28-32% |
| Carousel data: | |
| - Flow | 250-350 m3/hr |
| - pH | 7.2-7.5 |
| - Temperature | 20-24°C |
The results of the experiments are summarised in figure 4, wherein the percentage of H2S removal in the biogas scrubber is plotted as a function of the water flow/gas flow ratio.